Conventionally, technology has been developed in that an exhaust gas purifier (diesel particulate filter) is disposed in an exhaust gas flow path of an engine. The exhaust gas purifier includes an oxidation catalyst, a soot filter, and other elements that to perform purification treatment of exhaust gas discharged from a diesel engine (see, for example, patent document 1).
A conventionally known exhaust gas throttle device has been disposed in the exhaust gas flow path. This prevents a drop in a temperature of exhaust gas discharged to the exhaust gas purifier, thereby sustaining purifying capability of the exhaust gas purifier.
Conventionally, technology has been developed in that an EGR device is disposed in the exhaust gas flow path to discharge exhaust gas from the diesel engine.
An EGR cooler for cooling EGR gas is disposed in the exhaust gas flow path of the conventionally known engine. This causes a part of exhaust gas discharged from the engine to the exhaust manifold to be recirculated to the engine through an intake manifold, thereby minimizing the amount of emission of NOx (nitrogen oxide) of the engine. Further, an exhaust gas throttle valve is disposed to increase a pressure of exhaust gas of the engine, and the exhaust gas throttle valve increases the pressure of exhaust gas from the engine, thereby raising the temperature of exhaust gas discharged from the engine. This ensures a necessary temperature of exhaust gas for the exhaust gas purifier (such as the soot filter) to oxidize particulate matters (PM) collected from exhaust gas for removal.